This invention relates to soda-lime-silica glass particularly suitable for controlling transmittance of solar radiation in window glazing applications. The glass may be generally described as being green tinted, and is designed to have low heat transmittance and particularly enhanced absorption in the ultraviolet wavelength range. This is desirable for the sake of reducing the rate at which the sun's rays deteriorate plastics and fabrics in applications such as automobiles. A particular objective of the invention is to permit glass of this type to be made at a lower cost by reducing the amount of costly ingredients required.
Soda-lime-silica flat glass may be essentially characterized by the following composition on a weight percentage basis of the total glass:
______________________________________ SiO.sub.2 66-75% Na.sub.2 O 10-20 CaO 5-15 MgO 0-5 Al.sub.2 O.sub.3 0-5 K.sub.2 O 0-5 ______________________________________
Other minor ingredients, including melting and refining aids such as SO.sub.3, may also appear in the glass composition. Small amounts of K.sub.2 O, BaO or B.sub.2 O.sub.3 and other minor constituents have also sometimes been included in flat glass and may be considered optional. To this base glass are added the coloring constituents that produce the transmittance properties of the glass. The primary colorant in the category of glasses relevant to the present invention is iron, which is usually present in both the Fe.sub.2 O.sub.3 and FeO forms. As is conventional, the total amount of iron present in a glass is expressed herein as Fe.sub.2 O.sub.3, regardless of the form actually present. A typical green tinted automotive glass has about 0.5 percent by weight total iron, with the ratio of FeO to total iron being about 0.25.
Recently, it has become important to maximize the solar performance of automotive glazing. The use of larger glass areas and the elimination of CFC air conditioning coolants places a greater burden on the car interior and air conditioning systems. A goal has been established to limit ultraviolet transmittance to no more than 38 percent in some automotive glass. At the same time, it is a requirement that glass in vision areas of automobiles have an luminious transmittance of at least 70 percent.
Two approaches have been followed to improve the solar performance of the glass and meet these goals. In the first approach, high iron levels are used in the glass. The colorant composition and transmittance properties for examples of two commercial annealed products of this high iron, dark green tinted type along with an example of the conventional, light green tinted glass described above are set forth below:
______________________________________ Light Green Dark Green Dark Green Example A Example B Example C ______________________________________ Total Iron (wt. %) 0.521 0.803 0.728 FeO/Tot. Iron 0.268 0.284 0.291 LT.sub.A (%) 80.45 71.1 72.44 TSUV (%) 54.82 38.8 42.28 TSIR (%) 37.38 22.4 24.62 TSET (%) 57.85 44.5 46.92 ______________________________________
Although Examples B and C show a reduction in ultraviolet transmittance, the percentage still exceeds the desired goal. Merely increasing the amount of total iron to reduce the ultraviolet transmittance is not desirable because it would impermissibly lower the luminous (visible light) transmittance. Furthermore, the use of very high levels of iron can create problems in manufacturing the glass, such as shorter campaigns or the use of expensive electric boosting.
The second approach uses cerium oxide or cerium oxide plus titanium oxide in glass to reduce ultraviolet transmittance as disclosed in U.S. Pat. Nos. 2,860,059 and 5,077,133, and the following example is of a commercial product that takes this latter approach:
______________________________________ Example D ______________________________________ CeO.sub.2 (wt. %) 0.60 TiO.sub.2 (wt. %) 0.22 Total Iron (wt. %) 0.783 FeO/Tot. Iron 0.266 LT.sub.A (%) 72.5 TSUV (%) 31.8 TSIR (%) 23.7 TSET (%) 45.7 ______________________________________
This glass exhibits the desired combination of low ultraviolet transmittance and high luminous transmittance, but the high cost of cerium sources substantially increases the cost of making this glass. It would be desirable if these objectives could be met without incurring such high raw material costs.
It has also been found that glasses made according to these two approaches darkens upon tempering and exposure to solar ultraviolet radiation. This in turn lowers luminious transmittance. It is therefore necessary to reduce the heat absorbing component (Fe.sub.2 O.sub.3) to ensure the glass has at least a 70% luminious transmittance after use.
For these reasons, it would be desirable to produce glasses with enhanced spectral properties but at lower cost and with reduced solarization.
The transmittance data provided above and throughout this disclosure, except where noted, is based on a glass thickness of 3.9 millimeters (0.154 inch). Luminous transmittance (LT.sub.A) is measured using C.I.E. standard illuminant "A" over the wavelength range 380 to 770 nanometers at 10 nanometer intervals. Total solar ultraviolet transmittance (TSUV) is measured over the wavelength range 300 to 400 nanometers at 10 nanometer intervals. Total solar infrared transmittance (TSIR) is measured over the wavelength range 800 to 2100 nanometers at 50 nanometer intervals. Total solar energy transmittance (TSET) represents a computed value based on measured transmittances from 300 to 2100 nanometers at 50 nanometer intervals.
To determine this transmittance data, the transmittance values are integrated over the wavelength range [a,b]. This range is divided into n equal subintervals of length h by points {X.sub.o, X.sub.1, . . . , X.sub.n } where X.sub.i =a+(i.times.h). Generally, either the Rectangular Rule or the Trapezoidal Rule is used to compute the transmittance data. For each method, a different interpolating function is used to approximate the integrand .function. in each subinterval. The sum of integrals of these interpolating functions provides an approximation of the integral: ##EQU1##
In the case of the Rectangular Rule, the constant value .function.(X.sub.i) is used as an approximation of .function.(X) on [X.sub.i-1, X.sub.i ]. This yields a step-function approximation of .function.(X) on [a,b], and the numerical integration formula: ##EQU2##
For the Trapezoidal Rule, .function.(X) is approximated on [X.sub.i-1, X.sub.i ] by a straight line passing through the graph of .function. at the end points. Thus, the interpolating function for .function.(X) is piecewise linear on [a,b] and the integration formula becomes: ##EQU3##
The transmittance data presented throughout this disclosure is based on the Trapezoidal Rule.